Don -- The earliest Githens in my family line that I have found is a James Githens (wife Caty) living in Bourbon/Nicholas county KY who was born probably around 1755. I haven't connected him to any of the "original" Githens men in the family history by Sherwood Githens (The Githens Family in America). James had at leasttwo sons, Henry (wife Rebecca Smith) and John, and I believe a daughter Elizabeth, who married Matthew Stewart. I am descended from Henry's son Valentine (wife Martha Thomas), who moved up to Indianapolis IN. This family is recorded with an amazing variety of spellings of Githens, including Gittings, Gethings, and even Gidions. I have a lot of information about Henry Githens and his descendants, and a little bit about James. If you are interested, I would be happy to share it.
Do you have any information about the John Githens born 1720 besides his appearance in some posted family trees? I am also curious about the 1678 will you mentioned. Where was it executed?
Have you checked the Sherwood Githens book? I am inclined to be very cautious with the information he offers, since he attributes my several-greats-grandfather Valentine's children to an Edward Githens. I have just discovered that a small library in a town not too far from where I live now has a copy of The Githens Family in America, and I think I will go there next week to see if I can get any new leads. Let me know if you would like me to check any Githens folk listed in that book.